Supervisors: Dr Alejandra Sanchez-Franks,  Dr Bengamin Moat, Dr Ruza Ivanovic, Dr Chris Banks, Dr Joël Hirschi

The Atlantic meridional overturning circulation (AMOC) is a large-scale ocean circulation system comprised of currents that carry warm, shallow water northwards and return cold deep-water southwards. The AMOC is crucial in maintaining the relatively mild winter climate of Northwest Europe.  A shutdown of the AMOC would strongly impact European temperature and precipitation variability.

Meridional coherence is the idea that the AMOC functions as a coherent flow across the Atlantic basin (e.g. changes observed in the AMOC at one latitude will also be observed at other latitudes). It is generally accepted that the AMOC is meridionally coherent on long (decadal+) timescales [1]. On shorter timescales, it is not well understood where and how the meridional coherence breaks down, how the associated driving mechanisms change, or how these influence climate.

The primary objective of this project is to use new techniques combining in situ and satellite data to investigate these questions. You will then use these observations to assess state-of-the-art climate model projections. This will lead to a better understanding of the influence of the AMOC on European climate.

Methodology: RAPID is an international campaign that has been monitoring the AMOC since 2004 using a moored instruments array along a line at 26ºN. Techniques have been recently developed using satellite altimetry and gravimetry to estimate the AMOC strength at the RAPID line [2,3]. You will use the techniques developed at 26ºN to obtain direct estimates from satellite [2,3], providing a basis for extending the AMOC to the north and south of the RAPID line. This will enable you to reconstruct the first whole basin AMOC time series spanning 34ºS to 58ºN from satellite data alone. You will learn how to use both gridded and along-track sea level products. Gridded products (altimetry and gravimetry) will be used to provide a first estimate of a satellite-based AMOC across different latitudes. Higher resolution along-track satellite data (e.g. Jason, Sentinel-3A/B, Sentinel-6, CryoSat2) will be used to compare SSH data with RAPID moorings particularly near the coast (<50km), where the SSH signal is not very well resolved with gridded satellite products [2]. The approach will incorporate both standard altimetry products as well as higher resolution SAR altimetry and there is the potential in the latter part of the studentship for inclusion of data from the innovative SWOT mission. The incorporation of higher resolution along-track altimetry will provide significant improvements on existing methods to estimate AMOC strength, where coastal variability has been poorly resolved [2]. Other mooring arrays such as MOVE (16ºN), OSNAP (58ºN) and SAMBA (34ºS) will then be used to provide validation for the satellite-derived estimates of the AMOC. For latitudes in between the mooring arrays, density profiles from Argo floats (autonomous ocean profilers) will be used to fill data/knowledge gaps and provide further validation for the satellite derived AMOC estimates.

 

You will use the resulting latitudinal estimates of the AMOC to investigate meridional coherence of the AMOC variability from annual to decadal time-scales. The overarching objective will be to assess the nature and evolution of the AMOC meridional coherence: where it holds, where it falls apart, its drivers and wider implications these results have for the North Atlantic climate system. Based on this new understanding, you will evaluate the ability of climate simulations (e.g. CMIP6) to adequately represent the observed AMOC characteristics and their connection to European climate.

Training: The student will be hosted at the National Oceanography Centre (NOC), Southampton, and registered at the University of Leeds. The student will be working closely with supervisors at the NOC and the University of Leeds.

Specifically, you will be trained in:

• Use of satellite altimetry and gravimetry including in the coastal zone – data handling, analysis, and interpretation.
• Ocean observation techniques, which will include the opportunity to take part in international research expeditions. More information on our research expeditions can be found here: https://noc.ac.uk/science/previous-expeditions
• The use of high-performance computing for analysing ensembles of climate simulations.
• You will be part of the RAPID team and have access to the expertise of a diverse and international group of scientists within the NOC and the University of Leeds.
• You will also benefit from the experience of scientists working on EPOC (Explaining and Prediciting the Ocean Conveyor) – a large Horizon Europe project which is part led by NOC. EPOC runs from 2022-2027 with the goal to exploit new observational efforts and the latest modelling advances to provide a new conceptual framework for the AMOC.
• Communication skills (oral, written, presentation and academic networking). The RAPID group and EPOC project have several international collaborators, and you will be encouraged to participate and present at national and international workshops and conferences.

References:

[1] Bingham, R. J.,  Hughes, C. W.,  Roussenov, V., and  Williams, R. G. (2007),  Meridional coherence of the North Atlantic meridional overturning circulation, Geophys. Res. Lett.,  34, L23606, doi:10.1029/2007GL031731.

[2] Sanchez-Franks, A., Frajka-Williams, E., Moat, B. I., and Smeed, D. A.: A dynamically based method for estimating the Atlantic meridional overturning circulation at 26° N from satellite altimetry, Ocean Sci., 17, 1321–1340, https://doi.org/10.5194/os-17-1321-2021 , 2021.

[3] Landerer, F. W., Wiese, D. N., Bentel, K., Boening, C., & Watkins, M. M. (2015). North Atlantic meridional overturning circulation variations from GRACE ocean bottom pressure anomalies. Geophysical Research Letters, 42(19), 8114-8121

Jason

Jason and Sentinel-3 A/B

Jason, Sentinel-3 A/B and CryoSat-2

 

Unlike other altimeter missions, CryoSat-2 has a long repeat orbit and can therefore retrieve data over far more ground locations. This is shown in the figure above where the tracks of the Jason series, Sentinel-3A/B and CryoSat-2 are shown. The top image just shows the tracks of the Jason series with a repeat orbit of about 9.9 days (any point on the Earth’s surface is passed over every 9.9 days). The second image shows the tracks for Jason but also with the tracks from Sentinel-3A/B . There are more tracks for Sentinel-3 but the repeat orbit is now about 27 days. Finally, the CryoSat-2 tracks are added in the last image, showing a high density of tracks.

Moorings deployed in the North Atlantic from RAPID research expeditions. Image credit: Ben Moat.